THE HERO of The Whole Truth learns that in politics it does not pay to tell the truth, and that the biggest liars and the most vigorous defenders of lying reap the greatest profits. Most Americans were already quite aware of that, having watched in wonder as the perfidies of Watergate rained gold upon the lecturers, script consultants and authors of the Nixon gang -- including, of course, as herewith demonstrated for the second time, John Ehrlichman himself.

The first 100 pages or so of this novel are the equivalent of a stretch in a federal minimum-security prison -- no great hardship, but time certainly does drag. However, when Ehrlichman has finally finished putting together the ingredients of the plot's crises, the rest of the book marches smartly along.

Robin Warren (ex-jock, Stanford law grad, handsome but not profoundly smart) gets a job in the administration of President Hugh Frankling. Warren is a fancy flunky, handling White House liaison with businessmen. He falls under the influence of one multinational -- who seems to be a cross between Robert Vesco and Harold Geneen. This fellow, using Warren for entree, persuades President Frankling to set up a CIA operation to assissinate the leaders of Uruguay and replace them with politicians less likely to expropriate U.S. holdings. Frankling gives the go-ahead during a very drunken meal aboard the presidential yacht. The operation, needless to say, not only fails but is exposed to the hisses and boos of the United Nations.

Frankling fears that if his personal responsibility for the failed coup is established, he will be impeached and thrown out of the White House (this is the weakest part of the plot, expecting us to believe that the attempted murder of a few foreigners would upset Congress that much). So he blames it all on Warren, accusing him of acting alone. But Warren refuses to take the rap and discloses that he was just carrying out direct presidential orders.

Thus it comes down to what might appear to be a lopside struggle between the president and his aide, each attempting to establish his own credibility and to destroy the other's. Who will win? The battle rages through the press and through the Senate's Urugate hearings, and is replete with sex and bribery and other assorted pastimes. (Text omitted) or by reflection of his own, Ehrlichman is sometimes almost as good as Robert Benchley in delineating the contours of the mediocre mind: "When he had dressed, Hugh Frankling took the Uruguay file... and read the memorandum. ... It was all about Uruguay, of course." He sometimes has the sarcastic snap of a good mystery writer: "Frankling was using the genial, deep voice usually reserved for large contributors and children under the age of eight." Even at his worst he sounds no worse than, say, Mickey Spillane warmed over: "She wanted him naked and hot and with that odor which was uniquely Robin at stud."

But what gives The Whole Truth its real vitality is, of course, the Watergate in it. To Ehrlichman, the Watergate experience is like "starter" to a sourdough baker; each new loaf includes some of the old dough. He used Watergate starter in his first novel, The Company , and here it is again, including a number of Nixon-era clones: the wiry, gauntfaced, sharp-voiced first lady; the attorney general's hard-drinking, loud-mouthed but oddly appealing (or at least pitiable) Southern-belle wife; the president's unconscionable top assistant who looks upon his boss as infallible and whose eight assistants comprise the dangerously predatory "Beaver Patrol."

President Frankling is more and less than a Nixon, but when he goes on television in a final desperate effort to save himself, one hears a nostalgic echo of the Dickie Bird rhetoric: "I'm not a quitter. And my wife, Gloria, is not a quitter. After all, her name was Gloria MacDonald, and, you know, the Soctch never quit." One is also treated to an echo of the Nixon-tape tome: "I'm going to have this lovely meeting at Blair House for everyone and Secretary -- uh -- the wop at Welfare..."

An advance blurb says the book contains "sensational revelations that will make headline news." Well, only if the reader risks translating innuendo into fact, or is willing to conclude that the acts of bribery, chicanery, press manipulations and conflicts of interest crammed into this supposedly fictional work are based on actual events that Ehrlichman participated in or knew of during the Nixon years. President Frankling sends aides to the chief justice to discuss cases and to persuade him to twist the arms of associate justices; the president is assured that the chief justice is continuing to prove his gratitude for being elevated to the Supreme Court. Are we supposed to read Warren Burger into that? Are we supposed to read Chile for Uruguay and to take it that Nixon was drunk when he launched the CIA on that little misadventure? For the fun of it, I'm willing to make those leaps. But before Ehrlichman and his promoters start asking for headline space, he should step out from behind the devious protection of a very ordinary novel and talk straight for once. CAPTION: Picture, John Ehrlichman, AP